4. Inspect the working surfaces of the brushes
paper and a brush seater are the best tools you can use
occasionally and keep the full surface bearing on the
to accomplish a true seat. The following paragraph
rings. To prevent the formation of brush slivers, make
describes the method you can use to seat the brushes of
sure the brushes do not extend beyond the edges of the
a generator by using sandpaper.
rings.
First, disconnect all power from the generator. You
Scoring of the collector rings is usually due to hard
must take every precaution to make sure the generator
particles that become imbedded in the brush contact
will not be accidentally started while you are performing
surfaces. This condition is also caused when the
brush maintenance. Next, lift the brushes to be fitted and
incorrect grade of brush is used. Correct scoring by
insert (sand side up) a strip of fine sandpaper between
resanding and refitting the brushes or by changing to the
the brushes and the collector rings. Hold the sandpaper
correct grade of brush.
tightly against the collector rings. Allow the brushes to
Flat spots or pitting on the collector rings develop
be held down by normal spring pressure. Pull the
from many causes. Black spots can also sometimes
sandpaper in the direction of normal rotation of the
appear on the collector rings. These are normally not
generator, as shown in figure 5-28. When returning the
serious, but they should be removed immediately (at the
sandpaper for another pull, lift the brushes. Repeat this
first securing of the generator). Rub the collector rings
procedure until the brushes are accurately seated.
lightly with fine sandpaper to remove these spots.
Always finish with a finer grade of sandpaper. Use a
P i t t i n g sometimes develops because of the
vacuum cleaner during the seating operation to prevent
electrolytic action on the surface of the rings caused by
current flow. This pitting is general over the entire ring
After sanding, use a vacuum cleaner to clean the
area and does not cause localized flat spots. When this
collector rings and windings to remove all carbon dust.
condition occurs, reverse the polarity of the rings. Leads
to the collector brushes or at the switchboard should be
Collector Rings
made long enough to permit this reversal of polarity.
Reversal of the polarity of the rings will in no way affect
The collector rings on an ac generator should be
the phase rotation of the generator.
given careful attention. The following procedures will
help you to maintain good, polished surfaces:
TROUBLESHOOTING
PROCEDURES
1. Inspect the brushes regularly to see that they
Though proper preventive measures are taken on
move freely in their holders.
m o t o r s and generators, sometimes the GSE must
2. Keep the rigging free from dust, oil, salt, lint,
troubleshoot these machines. Three of the most recurrent
metal particles, and dirt.
electrical troubles in motors and generators are grounds,
open circuits, and short circuits. Because you, as a GSE,
3. Brushes need no lubrication, so you should keep
will have to locate and repair these faults, we will discuss
the rings free from coating and scaling of any kind by
each of these faults in the following sections.
cleaning them periodically.
Grounds
A ground on a motor or generator is a zero or
low-resistance path that is caused by a breakdown in
insulation. This condition extends from ground to a
winding or some other conductor in the machine. If no
other part of the system is grounded, a single ground in
any of the windings of a machine will cause no particular
harm to the machine. If, however, the machine and its
connected circuit have two zero or low resistance
grounds at points of different potential, the result will be
similar to a short circuit. This condition could cause
considerable damage. This danger can be avoided by
Figure 5-28.--Method of sanding brushes.
keeping the machine and its connected circuits free from
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